After re-reading your original post ...
Confirm your test results with your LFS.
First find out why your readings are so high. Test your newly mixed water before use to establish what you're adding to your tank; ie Alk Ca and Mag.
As Warren stated, stop dosing now if you currently are.
Perform some more 'large' water changes with new water that is closer to ideal parameters. But, do so knowing what you are adding to your system. Read on ...
This next part answers your question about your high Ca. I know of nothing you can do aside from water changes that will safely drop it besides your corals consuming it.
Again, the best way to drop the levels of your Ca and Mag is using new water with lower levels.
And to answer your question about your corals, just leave a couple or few days between water changes so they have time to adjust to the drop in levels.
Some math for you, 'cause it's what I'm famous for
You're going to want to know how the new water will impact your current water. This will dictate how much water you'll change at one time.
Use My formula below. Click the link and enter your params in place of the letters. Only change v w x y and z.
DO NOT CHANGE ANY OF THE CHARACTERS. This will basically Destroy the formula
And to make this stupid simple, Yes it works with almost ANY test param.
Alk, Ca, Mag, NO2, NO3, PO4 etc. If it can be divided, it works.
((V*W)+(X*Y))/Z.
V=Amount of existing tank water left behind during water change. IE; 100g system and 25g WC, your answer is 75g.
W=Current test parameter of tank water.
X=Amount of new water to be added to tank.
Y=Current test parameter of new water.
Z=Total tank/sump volume.
So, if you have 100 gallons of total water, and perform a 25G WC, V = 75 and X = 25.
And if your tank reads, say, Ca 410, and your new water reads 260, W = 410 and Y = 260.
For Z, you MUST know the total system volume!!!!
I double checked my formula through Google and it's good. Just be sure to enter stuff correctly.
If you want to confirm it, my control numbers result in 418.91, and are listed below.
V = 90g Old water
W = 420 Ca in old water
X = 25g New water
Y = 415 Ca in new water
Z = 115g Total system volume